Moderate concentrations of silicic acid are present in hot, pressurized, aqueous, geothermal fluid as a result of the dissolution of silica containing minerals from the rock material in contact with the geothermally heated water while it is still underground. This fluid may be discharged from normal surface manifestations or may be produced artificially, from wells drilled into suitable formations, which increase mass withdrawal, for whatever purpose. Usually, the purpose of drilling geothermal wells and producing the hot geothermal fluid is to provide an energy source for the production of electric power.
If silicic acid concentration in the water produced at the surface is sufficiently high, when the temperature or the pressure of the water is reduced, polymerization, with spontaneous homogeneous nucleation, of the siliceous values in the produced water results in the formation of critical size nuclei, which rapidly grow to form primary particles of silica. Spontaneous nucleation of the parent monomeric silicic acid species, and subsequent growth thereof by polymerization, forms polymeric primary silica particles which attain a physical dimension of about 1.5 nm.
Under slightly acidic to neutral conditions, this is best represented by the condensation reaction: ##STR1##
Under more alkaline conditions, e.g. pH&lt;7, dissociation of silicic acid to silicate ion becomes significant, according to the equation: EQU H.sub.4 SiO.sub.4 .revreaction.H.sub.3 SiO.sub.4.sup.- +H.sup.+
Polymerization under these conditions is considered to favor the following reaction: ##STR2##
Primary particles resulting from this Type of reaction can be observed under a high power transmission electron microscope, as shown in the attached drawing, for example FIG. 1.
In this invention, the tertiary aggregate amorphous silica products are differentiated by structure into three (3) classifications, namely: Types I, II and III. Type I silica, as that term is used and defined herein, is unique, but may be considered to be somewhat similar in microstructure to the silica which is commonly prepared commercially by acid treatment of sodium silicate solutions. Such sodium silicate solutions are commonly prepared by digestion of quartz sand with sodium hydroxide. Type II silicas, as the term is used and defined herein, is also unique, but may be considered to be somewhat similar in microstructure to a class of commercial silicas which are conventionally prepared by the so-called "arc" process, that is by the oxidation of silicon monoxide under anhydrous conditions at very high temperatures. Type III silicas are somewhat of a cross between Types I and II, as will hereinafter be more fully explained.
Reference is made to FIG. 8 hereof. This figure shows a transmission electron photomicrograph of a naturally occurring geothermal silica deposit. The silica shown in this figure is to be contrasted to the silicas shown in FIGS. 1-7 which are Type I, II and III silicas, as indicated on the figures, made according to the practice of this invention.
All of these novel types of tertiary aggregate amorphous silicas are recoverable from geothermal water by the practice of this invention by controlling the conditions of polymerization of the silicic values contained therein, the conditions of the growth of the primary particles of polymerized silica, and the conditions of precipitation of the polymerized silicas. The controls which are required will be detailed hereinafter.
Geothermal waters vary from location to location. However, such waters can be generically characterized as having a neutral to slightly alkaline pH. In addition to the dissolved siliceous values, mainly silicic acid, these waters often have significant concentrations of other ions, such as, for example: sodium, potassium, lithium, magnesium, rubidium, cesium, boron, chlorine, and arsenic. These waters also often contain dissolved gaseous species, such as, for example: H.sub.2 S, CO.sub.2, and NH.sub.3. Table 1 below shows typical chemical analyses of representative geothermal waters from several wells in the same field.